Electric induction heating



June 23, 1942. V 2,287,542

ELECTRIC INDUCTION HEATING Filed Nov. 8, 1940 TIMER \o I H) 3 x \e J:53.1.

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Patented June 23, 1942 ELECTRIC INDUCTION HEATING Alfred Vang, Detroit,Mich., assignor to Clayton Mark & Company, Evanston, Ill.

Application November 8, 1940, Serial No. 364,770

4 Claims.

This invention relates to electric induction heating, and moreparticularly to apparatus for translating power from commercial supplylines to high frequency application circuits for ab sorption in work asheat.

One object of my invention is to provide a power translating circuitwhich will convert power at commercial frequency to power at highfrequency with a minimum of loss in the conversion.

Another object of my invention is to make a simple apparatus ofrelatively compact dimensions for providing a relatively great amount ofhigh frequency electric power.

An ancillary object of my invention is to provide a low cost source ofhigh frequency electric power.

Another object of my invention is to provide a dependable source of highfrequency power which shall be under accurate and flexible control.

A further object of my invention is to provide an apparatus fortranslating power which will have a. minimum rate of deteriorationrelative to the amount of power translated.

A still further object of myinvention is to extend the useful range foremployment of induction heating by providing a method for translatingpower to high frequency, of which practical embodiments can be madeeconomically in a wide range of sizes.

Another object of my invention is to provide a source of high frequencypower which will require a minimum of skill to operate.

Other objects and advantages of the invention will become apparent asthe specification proceeds.

In the drawing in which similar numerals indicate corresponding parts:

Figure 1 is a wiring diagram of an embodi ment of my invention forconverting one half of the supply cycle.

Figure 2 is a wiring diagram of an embodiment of my invention forconverting both halves of the supply cycle.

Figure 3 is a wiring diagram of an embodiment of my invention employingthe discharge tube described in my copending application, Ser. No.364,701, filed Nov. '7, 1940, for Discharge tubes.

Referring to the drawing, and more particularly to Figure 1, the numerall indicates a transformer for raising the alternating potential of thesupply lines II and I! to a predetermined voltage, for charging thecondenser H with an increment of power, which will be discharged in theform of resilient oscillations at high frequency through the inductancell by closing the high frequency switching means I5. The discharge ispermitted to reverse itself immediately so that energy which remains inthe high frequency circuit after the first passage therethrough, in theform of an opposite charge of the condenser, is permitted tore-circulate, or pass through the switching means in either directionuntil it is entirely dissipated. This is termed discharge by resilientoscillation, distinguishing the present invention from inverter circuitswhich terminate the discharge after a single passage. Thus my switchingdevice permits a greater part of the energy to be absorbed in the workthan is the case where the switching means operates in a uni-directionalmanner, either always in the same direction, or serially in differentdirections. High frequency power may be taken from the high frequencycircuit I 0 in any conventional manner, as, for example, by insertingmetallic objects to be heated into theoscillating field of the coiledinductance H, or by coupling a secondary high frequency circuit (notshown) with the circuit It in any conventional manner.

After the energy of the discharge has been dissipated from the circuitIS, the condenser will charge again, and discharge as before,transferring another increment of energy to the high frequency load. Theoutput wave form is thus similar to that of oscillators for damped wavewireless telegraphy.

The operation of the switching means I5, however, distinguishes thepresent invention from the circuits used in damped wave wirelesstelegraphy, in that a controlled ionic discharge through a low pressuregas replaces the conventional spark gap; but it is the spark gap whichhas limited the use of the damped wave oscillator for induction heating,because the gap has a relatively high resistance, causing a loss ofnearly half of the power of the high frequency circuit in the gapitself, which in turn creates a very high local temperature, resultingin a gradual disintegration of the gap. Both of these factors reachseriously detrimental proportions in induction heating, where the amountof power involved is often relatively great.

In the present invention, the power loss is reduced virtually to aninsignificance by the use of a switching means employing gas under lowpressure as a conducting medium. The gas (preferably mercury vapor) hasa relatively high resistance when not ionized, but a relatively very lowresistance when ionized, which properties are consequent upon thereduction of pressure, as is well known in the art.

Thus, the low resistance of the switching means l5 during the conductingperiod greatly reduces the amount of heat generated within the means,

having the simultaneous effectsof increasing efficiency and increasingcarrying capacity, while reducing deterioration.

A further distinction between my invention and damped wave wirelessoscillators lies in the provision of control means, by which ionizationmay be started at will, independently of the potential across theswitching tube. This permits the same apparatus to be used over a verywide power range, wherein the power made available to the high frequencycircuit may be varied either by controlling the switching voltage atwhich discharge will be initiated, thus controlling the size of thecondenser energy increment, or by timing the discharge to take place ata given stage of the supply cycle, thus controlling the amount of powertaken from the transformer.

Switching means for the circuit, according to my invention, may have avariety of embodiments. in practice, although it is not ideally suitedto this use, is the conventional ignition-controlled cold-cathodemercury vapor rectifier tube, which does not behave as a rectifier in mycircuit, be-

cause it has a de-ionization time which is longer than one half cycle ofthe high frequency discharge, causing the tube to flash back or continueconducting in either direction until the condenser energy is nearly orentirely dissipated. The tube is illustrated in Figure 1, where I1 isthe envelope, l8 the anode, IS the mercury cathode, and 20 the igniterelectrode. The use of this conventional rectifier tube to conductcurrent in both directions is new in my invention, and constitutes agreat improvement in the art. It is wearing, however, upon theparticular tube described, and hence a difierent embodiment can be usedto greater advantage, as will be described presently.

To initiate ionization at the proper time, a

timer 2la is employed to synchronize the initiation of ionization withone half cycle of the supply potential, connections vas shown, or of thecondenser potential, connections not shown. The timer circuits willnecessarily vary with the application, and neednot be shown in detail,as they are known, or can readily be devised by anyone skilled in theart.

The low voltage line H, II, and the transformer I0 can be replaced by ahigh voltage line,

provided suitable impedance is included to protect the line. Also, inthe same manner, the low voltage line could be used directly, but thenthe apparatus would lose efficiency, as the increment of energy absorbedby the condenser would be smaller relative to the ability of the line todeliver power. Hence, it is in general preferable to use a relativelyhigh voltage to avoid excessive cost for condensers.

Figure 2 shows how both halves of the supply cycle can be drawn upon bya circuit otherwise identical with that of Figure 1. To accomplish this,another rectifier tube Ila of the same type has been connected inopposed parallel relation to the first. The only other changes necessaryare to add to the timer connections 2lb to accommodate the new tube, andto take customary precautions for the passage of a heavier currentthrough the remainder of the circuit.

As mentioned before, I use the rectifier tubes One embodiment which hasworked well iii for conducting current in both directions, but withregard to the initiation of ionization they still behave as rectiflers.Thus the circuit shown in Figure 1 operates only on that half of thecycle which causes the mercury electrode It to be initially negative,and hence while the circuit is potentially capable of operating oneither half of the cycle, actually it only starts on one half.

The circuit shown in Figure 2 provides a symmetrical arrangement, sothat whichever side of the switching means is initially negative, onetube will be capable of immediate ionization. Hence the circuit startson both halves of the cycle, only one tube starting each time, and thattube carrying the discharge in both directions, because it has not hadtime to de-ionize before the high frequency has reversed itself.

An alternative arrangement is to employ a fast-acting timer circuitwhich will excite or add to the excitement of the gas in each tubewhenever the mercury pool thereof becomes or is about to becomenegative, thus the high frequency discharge will pass back and forththrough the tubes alternately, each tube conducting at least the greaterpart of the current always in the same direction. This arrangement willprolong the life of the conventional type of rectifier tube, as

it will minimize the harmful effect of the flash back.

It will, of course, be necessary to superimpose a low-frequency timerupon the fast acting timer to render the latter inoperative during thecharging period.

In order to obtain still better performance from my invention,I canreplace the two rectifier tubes shown in Figure 2 with one of thedischarge tubes described in my copending application, Serial No.

364,701, filed November '7, 1940, of which this ap;

plication is a continuation-in-part, and as shown in Figure 3. Here thetimer 2 lo may also include a connection for the grid, to assist inde-ionization, as described in said application. The symmetry of mydischarge tube adapts it to operation in the present circuit on bothhalves of the supply cycle, and the absence of solid electrodes permitsa much heavier current to be carried, along with a, longer tube life.

It is obvious that changes may be made in the form, construction andarrangement of the several parts, as shown,.within the scope of theappended claims, without departing from the spirit of the invention, andI do not, therefore, wish to limit myselfv to the exact construction andarrangement shown and described herein.

Having thus described my invention, what I claim as new, and desire tosecure by Letters Patent of the United States, is:

l. A device for translating electric power, comprising a source ofalternating current, a condenser connected across said source, a tubehaving two liquid electrodes, means for starting an ionized discharge insaid tube, and an inductor, said inductor and said tube being connectedin series across said condenser, said inductor and said condenser beingtuned to a frequency sufficiently high so that said tube conducts atleast one discharge in both directions during the deionization time ofsaid tube.

2. A device for translating electric power comprising a capacitor, meansfor charging said capacitor from a source of alternating current, adischarge tube having two liquid electrodes, means for starting anionized discharge in said tube, and an inductor connected in series withsaid tube and said capacitor, the de-ionization time of said tube beinglonger than one-half cycle of the resonant frequency of the seriescircuit while said tube is ionized, whereby the tube is adapted tocontinue conducting alternately in each direction until the energy ofthe charge of said capacitor is substantially dissipated as outputenergy.

3. A device for translating alternating electric power from low to highfrequency, comprising a capacitor, means for charging said capacitorwith power of said low frequency, an inductor, a bi-directional mercuryvapor discharge tube having two separated mercury electrodes, adischarge-starting electrode adjacent to each of said mercuryelectrodes, and a timing device connected to said starting electrodesfor starting a discharge in said tube, said mercury electrodes,capacitor and inductor being connected in series to form a seriescircuit whose natural resonant period is not more than twice thede-ionization time of said tube.

4. A device for translating alternating electric power from low to highfrequency, comprising a capacitor, means for charging said capacitorwith power of said low frequency, an inductor, a bidirectional mercuryvapor discharge tube having two separated mercury electrodes, adischargestarting electrode adjacent to each of said mercury electrodesfor starting a discharge in said tube, said mercury electrodes,capacitor and inductor being connected in series to form a seriescircuit whose natural resonant period is not more than twice thede-ionization time of said tube.

ALFRED VANG.

